1. The Field of the Invention
The present invention relates generally to thermoelectric gas sample coolers. More particularly, it concerns a temperature controlled, thermoelectric gas sample cooler having a low mass, high-heat-flux unibody heat sink cooled by high-pressure turbulent air streams.
2. The Background Art
Incinerators, smoke stacks and other industrial equipment are known to emit large quantities of hot, vaporous gases into the atmosphere. In the interest of environmental protection and plant efficiency, such industrial gaseous emissions are frequently analyzed and evaluated as to compliance with governmental and in-house specifications. These gaseous emissions can be more effectively analyzed if the gas sample is transformed into a dry and cooled state. Of current interest are apparatus for quickly and efficiently removing heat from hot, gaseous emissions to thereby cool the gas to a predetermined temperature suitable for analysis.
Thermoelectric gas sample coolers are known in the art for cooling hot gas samples. These prior art coolers include a heat-exchanging tube through which a sample of hot, continuously-flowing gas is pumped. The heat-exchanging tube is thermoelectrically cooled and maintained at a predetermined temperature such that heat is removed from the gas as it flows through the tube. The heat is mechanically transferred to an air-cooled, metallic heat sink block by a thermoelectric element as known in the art. The heat sink block is large enough to absorb and release heat at a rate sufficient to enable the system to cool the gas along a substantial temperature range, for example from about 200.degree. C. to about 5.degree. C. The gas is gradually cooled as it flows through the tube such that it reaches the desired temperature just prior to discharge.
These prior art thermoelectric coolers are characterized by many disadvantages. The heat sink blocks are quite large, expensive, heavy and bulky due to the following structural particulars. Heat sink blocks comprise a plurality of metallic fins which are glued into a metallic support body to form a plurality of air-flow slots. The fins provide a substantial amount of surface area for contact with the cooling air to thereby accelerate heat removal from the heat sink. Although heat conductive glues can be used to glue in the fins, the glue is somewhat of a barrier against heat flow within the heat sink because the material properties of the glue are not entirely consistent with the metal from which the support body and fins are made. The obstruction to heat flow brought about by the glue requires the heat sink to be larger in order to absorb heat at a rate sufficient to maintain the thermoelectric element and the heat exchanging tube at desired temperatures.
Efforts to reduce the size of the prior art heat sink blocks are also limited by the thickness of the fins. If the fins can be made thinner, then the required amount of fin surface area can be achieved in a smaller heat sink block. However, if a smaller, more efficient heat sink is accomplished, the threat of overheating the thermoelectric element increases because a failure of the cooling air source would result in more heat "back-up."
The prior art gas sample coolers have traditionally been cooled with the use of "muffin fans" as are known in the art for moving large amounts of air at low pressures. The prior art teaches the use of "muffin fans" to suck air through the heat sink slots at low pressure to produce air streams characterized by laminar flow. Laminar air streams are cooler than turbulent air streams because mechanical mixing of the air raises the temperature of the air stream. The conventional wisdom of those skilled in the art teaches that the cooler, laminar air streams will be more effective in cooling the heat sink. Even if there were a prior art practice of applying nonlaminar, turbulent air streams, the fans used in the prior art cannot produce air flow at a pressure high enough to create turbulent air streams within the slots of the heat sink.
There is thus a need to achieve a gas sample cooler having a smaller, more efficient heat sink. There is a further need to provide a cooling apparatus small enough for use in conjunction with a smaller heat sink but capable of moving air at a pressure high enough to produce turbulent air streams within the slots of the heat sink. There is also a need to safeguard against the threat of overheating the thermoelectric element, a threat which is increased by a smaller, more efficient heat sink. Those having ordinary skill in the art will appreciate that these and other needs are met by aspects of the present invention.